Testing the generality of hemiparasitic plant effects on mesotrophic grasslands: A multi-site experiment

Restoration of species-rich, semi-natural grasslands is one of the key conservation challenges in Europe. Many of these grasslands have lost their diversity due to agricultural intensification. A new tool to facilitate restoration is to introduce hemiparasitic plants such as Rhinanthus species. R. minor accelerates restoration by decreasing the biomass of competitive plants and creating gaps which benefit colonising species. Questions remain about the generality of these impacts and whether R. minor can establish and facilitate community change in more productive grasslands. We addressed these questions by introducing R. minor into 13 mesotrophic grassland sites in southern England. These represented four main soil types and different ages of grasslands (eight permanent grasslands and five temporary ex-arable grasslands). We investigated two treatments using a factorial design: +/- Rhinanthus seed addition and +/- soil surface scarification. Rhinanthus populations established at all but one site. There was high variation in establishment success between the sites (range 0% to 25%), but productivity of the site or nutrient level did not affect establishment. However, scarification improved establishment. Rhinanthus decreased host (mainly grasses and legumes) biomass by between 27 and 37%. Scarification also decreased grass, legume, total and host biomass. There was a strong negative relationship between the abundance of hemiparasites and grass biomass. Non-leguminous herbs were not affected by the treatments, but grassland age had an effect as there were more herbs in permanent grasslands. The results show that it is possible to establish Rhinanthus into different mesotrophic grassland communities representing a variety of soil types and ages, suggesting it is a practical and widely applicable tool for restoration. The considerable variation in the rate of Rhinanthus establishment is unexplained. The factors behind this variation may lie in unfavourable weather conditions during germination and seedling stages, and herbivory.